The present invention relates to a RF amplifier used in a mobile communication system wherein a plurality of terminals are connected to each of base stations, and particularly to a RF amplifier suitable for use in a mobile communication system which simultaneously uses a plurality of carriers different in frequency between each base station and one terminal.
A current mobile communication system, which performs communications through the use of radio waves, adopts a cellular system capable of using limited frequencies with satisfactory efficiency with a view toward increasing subscriber capacity to be held therein. As shown in FIG. 14, the cellular system is one wherein a range of a radio wave (frequency f1) emitted from one base station 60a is localized by sharpening the directivity of an antenna 61a to thereby narrow a service area like 62a, whereby a plurality of base stations (corresponding to a base station 60b in FIG. 14, whose antenna and service area are designated at 61b and 62b respectively) spaced one base station or more away from the base station 60a can utilize the same frequency f1.
In the current cellular mobile communication system from the viewpoint of a demand for downsizing of a terminal, the amount of information transfer required by a system, and attenuation characteristics of an electro-magnetic wave in free space, the range of the radio wave emitted from the base station is set from a few km to about 20 km, the frequency to be used ranges from several hundreds of MHz to a few GHz, and radiation power of one terminal ranges from a few hundreds of mW to about 2W.
The number of subscribers to be accommodated, which is required for one base station, reaches from several tens to several hundreds of subscribers under such conditions. To this end, average transmitting power of from several tens of W to several hundreds of W is required as the output of a transmitting power amplifier of the base station.
Digital mobile communications are mainstream at present to realize a variety of communication services. Since a modulation system takes phase/amplitude modulation in this case, a transmitting power amplifier needs to have high linearity. While a high-power RF semiconductor is used in the transmitting power amplifier, the efficiency of the same power amplifier is inevitably reduced in the existing circumstances to compensate for its non-linearity. Incidentally, the required transmitting power ranges from several hundreds of W to a few KW as a saturated output.
Assuming now that RF power is transmitted from a transmitting power amplifier placed adjacent to a base station for indoor use, having a ground height of several tens of m to an antenna installed at a height of 100 m, for example, the loss of a RF cable for connection cannot be neglected. Namely, since a loss of about 5 dB to 6 dB is normally generated when the length of the cable is about 100 m, it is necessary to increase the output of the transmitting power amplifier to about five times as the loss-free case. Accordingly, the transmitting power amplifier of the base station is installed as close to the antenna as possible.
Various services for providing more amounts of information have been proposed to obtain many subscribers and improve the utility of the system. With a view toward implementing such services, a method has been discussed which simultaneously uses a plurality of carriers (multi-carrier) different in frequency between a base station and one terminal, thereby increases the capacity of a radio communication network equivalently, and performing the transfer of larger amounts of data. A configuration of the base station used in the current multi-carrier mobile communication system for providing such services is shown in FIG. 15.
A modem 63 for modulating and demodulating plural carriers is placed inside an office or building for a base station 60. A plurality of output lines of the modem 63 are connected to an amplifier of a transmitting signal and amplifier of a receiving signal 64 installed near an array antenna 61 placed in the rooftop of the building. The amplifier of transmitting signal and amplifier of receiving signal 64 is connected to the array antenna 61 through a RF cable 65.
Upon use of the plural carriers, an increase in the number of antennas for the base station with an increase in frequency and the placement of a RF (Radio Frequency) unit comprising the modem 63 in the base station 60 and the amplifier of transmitting signal and amplifier of receiving signal 64, etc. in plural form will cause problems such as ensuring of a construction place, an increase in the number of in-office devices, etc. Even in the case of a terminal for receiving signals sent from the base station 60, the provision of a transmitting circuit and a high-output power amplifier every different frequencies to transmit the plural carriers will lead to an increase in circuit scale, an increase in the volume of the terminal and a rise in the cost of each part, thus causing a problem.
Therefore, a system for amplifying a plurality of frequencies of carriers by the same RF unit has been discussed. Circuit configurations of a general base station and a terminal having adopted such a system are respectively shown in FIGS. 16 and 17. An amplifier of transmitting signal and amplifier of receiving signal 64 of the base station has a multi-carrier power amplifier (RF amplifier) 66 for simultaneously amplifying plural carriers and a low noise amplifier 67 for amplifying the received plural carriers.
A circuitry of terminal 68 included in the terminal has a baseband LSI (BB LSI) 69 for processing data to be transmitted and received, an analog front end 70 for modulating and demodulating the data to thereby generate a plurality of carriers, a RF/IF transmitting circuit 71 for amplifying the plural carriers outputted from the analog front end 70 and effecting frequency conversion thereon, a multi-carrier power amplifier (RF amplifier) 72 for amplifying a plurality of radio frequency carriers, and a duplexer 74 for sending a transmitting signal outputted from the RF amplifier 72 to an antenna 73 and simultaneously sending a receiving signal inputted from the antenna 73 to a RF/IF receiving circuit 75. The receiving signal outputted from the RF/IF receiving circuit 75 is brought to receive data by the analog front end 70, followed by supply to the baseband LSI 69.
A circuit configuration for reducing distortion by feedforward type feedback, which has been described in the specification of U.S. Pat. No. 4,580,105, for example, has been adopted for the multi-carrier power amplifiers 66 and 72 to amplify a plurality of carrier signals.
When signals having a plurality of frequencies are simultaneously inputted, RF amplifiers, i.e., multi-carrier power amplifiers for the current base station and terminal will produce unnecessary spurious signals called inter modulation spurious in the vicinity of the plural frequencies within a frequency band of a wireless system due to non-linearity of an amplification characteristic of a semiconductor. It is difficult to eliminate the spurious signals by an external filter because they appear within the frequency band of the system. Further, since the spurious correspond to spurious emissions, the spurious must be reduced to an allowable value or less by any means according to strict law""s control.
A limitation is imposed on the method of reducing distortion by the single amplifier like the above-described conventional circuit. It is thus difficult to sufficiently suppress the generation of the spurious signals. In order to realize services by the multi-carrier mobile communication system using the plural carriers, the development of a method for reducing spurious signals caused by a multi-carrier becomes an important problem in terms of the development of a cellular base station and terminal.
An object of the present invention is to provide a RF amplifier having a novel configuration, which is capable of suppressing the generation of spurious signals when a plurality of carriers are simultaneously amplified.
According to one aspect of the invention, for achieving the above object, there is provided a RF amplifier comprising dividers for respectively dividing each of a plurality of input signals different in frequency of carrier from one another into plural form, phase shifters for respectively assigning a weight of phase to each of divided signals corresponding to a number obtained by subtracting 1 from the number of divisions, a plurality of first combiners for respectively adding up the signals different in the frequency of carrier, out of the divided signals and signals each assigned the weight of phase, a plurality of amplifiers for respectively amplifying signals outputted from the plurality of first combiners, and a second combiner for adding the signals outputted from the plurality of amplifiers to output one signal.
Owing to the above configuration wherein the divided signals and the signals each assigned the weight of phase are amplified and added up, the spurious components are canceled out each other by the addition made by the second combiner to thereby suppress the generation of the spurious signals.
The principle of the suppression of the spurious signals will be explained below with reference to FIGS. 10 through 13. FIG. 10 shows the case where the number of carriers is two. Two signals having different frequencies of carriers, which have xcfx891 and xcfx892 respectively, are defined as input signals. First of all, the signal of xcfx892 is shifted by a phase xcfx86 with the signal of xcfx891 as the reference and inputted to its corresponding amplifier. Next, the signal of xcfx891 is shifted by the phase xcfx86 with the signal of xcfx892 as the reference and inputted to its corresponding amplifier. Since the amplifier comprises a semiconductor, the lowest degree of inter modulation spurious in inter modulation spurious produced in a system frequency, i.e., frequency components of the same spurious closest to the carrier frequencies as viewed from a frequency axis are given as 2xc3x97xcfx891xe2x88x92xcfx892 and 2xc3x97xcfx892xe2x88x92xcfx891. The difference in phase between these spurious signals becomes 3xcfx86. Therefore, if xcfx86 is set as xcfx86=60xc2x0 in advance, then third spurious will become zero in principle because of a phase difference of 180xc2x0 with respect to each other as shown in FIG. 11. On the other hand, the attenuation of a main wave reaches 0.38 dB.
FIG. 12 shows an example in which a carrier signal is divided into four. If xcfx86 is set as xcfx86=30xc2x0 according to calculations similar to the above, then third spurious will become zero and the attenuation of a main wave reaches 0.22 dB, as shown in FIG. 13. At this time, 3i (where i=1, 2, 3, . . . )th spurious become all zero in principle, and other degrees of spurious are given an attenuation of 6.5 dB. Inter modulation spurious components corresponding to an arbitrary degree to be noted can be cancelled out by increasing the number of divisions of an input signal, adding up signals each assigned the weight of phase after their division, and selecting a suitable phase difference xcfx86.
According to another aspect of the invention, for achieving the above object, there is provided a RF amplifier comprising s outer input ports for inputting s (where s: integer greater than or equal to 2) signals different in frequency of carrier from one another, s 1:m dividers respectively connected to the s outer input ports and for dividing the input signals into m (where m: integer greater than or equal to 2), sxc3x97(mxe2x88x921) phase shifters connected from second output ports of the s dividers to mth output ports thereof, s m:1 first combiners for inputting m output signals selected from s output signals sent from first output ports of the s dividers and sxc3x97(mxe2x88x921) output signals of the s dividers sent via the phase shifters and combining the output signals into one, s power amplifiers substantially identical in characteristic, for respectively amplifying the output signals of the s first combiners, and an s:1 second combiner for inputting the output signals of the s power amplifiers and combining the output signals into one, and outputting the combined signal to an outer output port, and wherein the m output signals inputted to the first combiners are different in frequency of carrier from one another, and the sxc3x97m output signals inputted to the s first combiners are different from one another.
According to a further aspect of the invention, for achieving the above object, there is provided a RF amplifier comprising s outer input ports for inputting s signals different in frequency of carrier from one another, s 1:2n dividers respectively connected to the s outer input ports and for dividing the input signals into 2n (where n: positive integer) sxc3x97(2nxe2x88x921) phase shifters connected from second output ports of the s dividers to 2 nth output ports thereof, sn 2:1 first combiners for inputting two output signals selected from s output signals sent from first output ports of the s dividers and sxc3x97(2nxe2x88x921) output signals of the s dividers sent via the phase shifters and combining the output signals into one, sn power amplifiers substantially identical in characteristic, for respectively amplifying the output signals of the sn first combiners, and an sn:1 second combiner for inputting the output signals of the sn power amplifiers and combining the output signals into one, and outputting the combined signal to an outer output port, and wherein the two output signals inputted to each of the first combiners are different in frequency of carrier from each other, and the snxc3x972 output signals inputted to the sn first combiners are different from one another.
Incidentally, the characteristics of the power amplifier using the semiconductor are brought to the following manner because the performance thereof might vary due to the temperature, a change in source voltage, etc. It is desirable in the latter two configurations that the phase shifters connected to the output ports of the dividers are defined as first variable phase shifters, and first variable attenuators are respectively connected to the output ports of the dividers, whereby the first variable attenuators are inserted between the output ports of the dividers and the first variable phase shifters, a second variable attenuator is inserted between one predetermined first combiner and the power amplifier and a second variable attenuator and a second variable phase shifter are inserted in series between the other first combiner and the power amplifier, a directional coupler is inserted between the second combiner and the outer output port, and a control circuit for adjusting the first and second variable attenuators and the first and second variable phase shifters through the use of a signal outputted from the directional coupler is placed. Control on the adjustment by the specific control circuit in this case is carried out to adjust the first variable attenuators and the first variable phase shifters, thereby minimizing spurious components existing in the final output of the power amplifier and to control the second variable attenuator and the second variable phase shifter, thereby maximizing carrier components in the output of the power amplifier.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.